Grind the walnuts in a mortar or in a chopper. Add them to the butter with the egg and flour. Knead a soft dough. Put it in cling wrap and shape it into a roll.
Place in the refrigerator for at least an hour and a half, and it is even better if you briefly put it in the freezer toward the end. Then it can be cut quite easily without breaking.
Once the dough is hardened, remove the foil and cut it into slices with about 1/2″ (1 cm) thickness. Arrange on a baking sheet and bake in a preheated 356°F (180 °C) oven for about 10 minutes.